Loop antenna circuit



July 15, 1941.

w. H. AGRIMDITCH Loop lANTENNA CIRCUIT Filed June 17. 1959 dr rare/vs er fau 00P Patented July 15, 1941 2,249,12e y Loor VANTENNA CIRCUIT wiuiamn. Gimditch, nydal, Pa., assigner, ilyl vmesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application June 17, 1939, Serial No. 279,745

(ci. 25o-33) 10 Claims.

This invention relates to loop antennas or aerials, and more particularly to loop antenna circuits capable of giving limproved performance and of being readily tuned over relatively wide frequency ranges.

In order to improve the portability of radio receivers and to simplify their installation it is desirable to employ tuned loop antennas inv place of the more usual and cumbersome outdoor antenna structures. As is well known in the art such a tuned loop `normally takes the place ofthe tuned antenna Vtransformer employed in the rst:

stage of 'conventional r-adio receivers. It has been found, however, that if the same tuning condenser be employed, the Ytunable frequency range of the tuned loop will be substantially less thanV that of the tuned antenna transformer. This effect has been traced to the increased distributed capacity between turns vand increased distributed capacity to ground of the loop antenna as compared to the vcorresponding capacities encountered in the tuned transformer. Obviously this dijiculty might be met by tuningtheloop with a variable condenser having a greater ratio of maximum toV minimum capacity.' Practically this means a larger and more costly condenser, and

therefore such a solution is not a desirable'one Where receivers are made in large quantities.

'I'he extended frequency range of'modern receivers increases the above-mentioned diiculty and makesv itl even Vless desirable to employ a larger tuning condenser; Although the broadcast band now used in the United States extends fromv about 550 to 1550 kc.; it has been common prac'- Vtice Vfor various reasons to extend the broadcast` range of radio receivers somewhat beyond theseV limits.` For example, itmay be desirable to in' clude in this band signals from so-cal1ed"wireless record players `which may be adjusted to 530 or 1600 kc., or to receive the police signals'in the band above 1600 kc.

With the above-considerations in mind, Vit is an important object `of vthis invention to provide novel means for -extending the tunable frequencyv range of Vloop antennas without Vthe necessity of employing specialor-costly tuning'condensers. vAnother objectof the invention is to provide means for minimizing the Vundesirableeiects of distributed intertur-n and ground-:capacities in loop antennas employed in-radio reception.

A further object of the invention is to provide `a tuned receiving loop antenna circuit having an unusually -high Q, or ratio of reactance to resistance.

Still another object of this invention isto materially improve the efficiency of all-wave radio r'eceivers by making possible the use of a smaller tuning condenser, thus permittingthe use of la larger loop in the short wave band with a resultant increase in signal voltage pick-up in that band. Y Y

The invention may best be understood by reference'to theaccompanying drawing, in which Fig. 1 is a diagrammatic illustration of oney embodiment of the invention, and

Fig. 2-is a group of generalized` performance' Although for the purposes of this invention,-v any form of balanced or unbalanced loop antenna may be employed, it Vispreferred'to Aemploythe shielded loop antenna illustrated, which is dis-f closed and` claimed in my copending application, Serial'No. 277,550 filed June 5, 1939. Such a loop antenna may comprise" a loop l comprising a` plurality of turns wound on a suitable frameworkfor support, and a shieldwinding 2 .surrounding the loop antennaand serving to shield the same'from Y nearby electrostatic disturbances; The electricall center of theshield'winding "2 is preferably'- grounded as indicated. The loop lmay be tuned u to resonance with a desired transmission by connecting across ita variable condensergwhich/ maybe one section of the tuning condenser. gang of a signal utilization means YIl. The-signal `uti' lization means may be a conventional superhete'r-' odyne radio receiver, and since its particulars'are n of no interest as regards 'this invention, only its; input circuit, comprising a vacuum tube fwhose grid and cathode electrodes are connected across' the tuning condenser 3, is shown. In the interests of simplicity, no biasing means sare shown.

Disregarding for the moment the' inductor Band supposing that the inner turn of loop l were lconnected directly to the grid of tube 5, it has been found that in view of the relatively large dis'4 tributedloop' capacity, the overall frequency range obtainablefby'varying the capacity'of thereondenser 3 between its maximum and minimum capacitiesA is very substantially less than would be obtained rwith the usual `tunedantenn'aV trans-s former energized, forV example, from afconventional flat top antenna system. The distributed capacity of the loop may be represented byan equivalent lumped capacity 1 in shunt with the loop, and hence in this case with the tuning 'condenser'3. Thus, it is obvious that-the'minimum tuning capacity of the circuit-will be verysubry stantially increasedY by the -distributed loop cai pacity andthe ratiobetween maximum and minimum capacity will be reduced.' They resulting narrowing of the frequency range over which the loop may be tuned is highly undesirable in modern radio receivers, as hereinbefore shown.

According to the present invention, the number of turns in the loop l is reduced, while on the other hand the loss in loop inductance is regained by the series insertion in the loop circuit of a substantially equivalent lumped inductance 6. By suitable design and placement in the receiver, the distributed capacity of this element to ground may :be made negligible as compared to the distributed capacity of the loop turns which it is designed to replace. In the preferred embodiment of the invention, the lumped inductance 6 is inserted in the high potential lead from the loop; thus, as shown in the drawing, the inductance 6 may be connected between the ungrounded end of the loop and the grid of the vacuum tube 5.

The eiect of this connection is, rstly, toreduce the distributed capacity of the loop by reason of the fewer turns employed, and, secondly, and more important, to greatly reduce the eiect of the distributed capacity referred to the terminals of the whole resonant circuit, i. e., between the grid of tube and ground. This arises from the fact that in the circuit illustrated, the distributed loop capacity is isolated from the tuning condenser 3 by the inductive reactance of the lumped inductance 6.

Where the construction of the loop is such that some of the turn-s have more distributedY capacity to ground than others, it is preferred that the end of the loop having the greatest capacity to ground be grounded. Hence, `in the diagram the outer turn'of the loop, being more closely adjacent to the grounded shield winding 2, may be grounded, while the inner turn of the loop may be connected tothe grid of the vacuum tube 5 by way of the lumped inductance E.

Assuming that the voltage induced in a loop of givenY size .is directly proportional to the number of Aturns in the loop, it might at rst appear that the practice of the present invention would seriously limit the overall sensitivity of a receiver employing it. As will now be shown, however, the overall sensitivity of the receiver may actually be improved by the invention. a conventional loop is tuned by an ideal tuning condenser (one having no losses), the voltage built up across the condenser at resonance will be Q times the signal induced in the loop by the passing wave, where Q is the ratio of loop reactancev to resistance. For purposes of explanation, it may be stated that a representative loop of good quality, when installed in a radio cabinet,

may have a Q of about 100. On ythe other hand,

the Q of smallV loops, closely mounted adjacent a crowded radio receiver chassis whose metal parts extend well into the eld of the loop, may be of the order of 50, or even substantially less. Accordingly, if the turns removed from the loop according to the present invention are replaced by a lumped inductance having a Q which is substantially higher than that of theloop, say 150 or 200, the loss in induced loop voltage may be compensated for so as to give substantially the same voltage across the tuning condenser as beforegcor in some cases the voltage across the condenser may even exceed that obtained wi-th a conventional system.

Reference may be had to curves A and B of Fig.' 2` where the above mentioned relationships are presented graphically. The horizontal scale represents the number of turns on the loop as a percentage of therturns on a full loop, a full Where' loop being one in which the lumped inductance 6 is zero, all the loop circuit inductance being in theloop itself. At zero on the horizontal scale, the loop I is entirely replaced by the lumped inductance 6. Thus, at zero percent, the signal voltage developed across the condenser 3, or the relative gain, may be taken as zero, while the relative gain at i. e., with no lumped inductance, may be taken as unity.

The curve A is representative of the case where the Q of the lumped inductance 6 is three times the Q of the loop. From this curve it will be noted, firstly, that by removing about 30% of the turns from a full loop, the voltage across the condenser 3 may be increased by about 7% as a result of the improved overall Q of the loop circui-t, and this in spite `of the fact that the voltage induced in the loop by the passing wave is decreased by about 30% as a result of the reduced number of turns. Secondly, i-t may be observed that, with' half the turns of the loop removed, the gain, or voltage across the condenser 3, will be the same as that in the case of a full loop. Thus, it is possible to greatly reduce the effect of distributed loop capacity across the condenser 3, and simultaneously improve the loop selectivity as a result of the considerably improved Q of the loop circuit, without the slightest ity.

Curve B represents the case where .the Q of it will be observed that the fall in relative gainis comparatively small between say 60% and 100%. Even where the ratio of Qs is less than two, the advantages resulting due to improvement in selectivity and the decrease in effective distributed loop capacityacross the condenser 3 will often koutweigh any small loss in sensitivity which may obtain.

How the use of the lumped inductance 6 in the loop circuit permits the use of a smaller condenser 3 to cover a desired constant frequency band is best illustrated in curve C of Fig. 2. The horizontal scale again represents the percent of turns on the loop I, as referred to the total turns which would be employed if no lumped inductance were used. The vertical scale represents the ratio of maximum to minimum tuning capacities necessary to cover a desired frequency band, e. g., 550 to 1550 kc. Where no lumped inductance is employed, i, e., at 100% on the horizontal scale,

a tuning capacity ratio of about 10.2 is requiredto cover the desired band, whereas if 50% of the turns of the loop are removed and a lumped inductance employed to restore the lost inductance, a tuning capacity ratio of only about 811 is necessary.v Alternatively if the maximum to minimum capacity ratio of the tuning condenser is unchanged and the lumped inductance increased somewhat to give the proper frequency coverage, then the capacity and size of the tuning con# denser` can be materially reduced. Y

While this is important from an economic standpoint, it is also of considerable importance from the `performance aspect where the invention is employed in an all-wave receiver yhaving a short-wave receiving loop antenna. Referring again to Fig. 1, it will be seen that a short wave loop 8 has been provided, with a switching means S whereby the broadcast band loop Imay be disloss in sensitiv short wave loop 8 connected across the condenser in its place. The short wave loop may comprise a single turn of wire andvis preferably disposed in .a plane which is perpendicular to the kplane of the broadcast loop l, as described and claimed in-the copending application of David Grimes, Serial No. 277,276, filed June 3, 1939. Y ,As-'will be understood by those skilled in the art, the smaller the tuning condenser 3, i. e., the lower its maximum and minimum capacities, the greater will be the inductance of the short wave loop 8 required to cover a desired high frequency range. With a short wave loop, this increased inductance is best secured by increasing the physical Ydimensions of the loop.v This .results in correspondingly improved signal pick-up, `since the signal voltage induced in a loop antenna is proportional to the enclosed area of the loop. It has been found that the improved performance, thus obtained at short wave lengths is of considerable importance in the manufacture of a successful all-wave receiver. Where the symbol Q is employed in the present specification and in the claims, it is used in itspusual sense asa figure ofv merit of the coil (loop or inductor) to which it is applied; i. e.; Q is equal to the reactance of the coil divided by its resistance, or

In a physical embodiment of the invention, as described herein, the lumped inductance 6 was wound upon a coil form and was supplied with a high quality powdered iron core to give a high Q. The inductance of the loop was about 110 microhenries, while that of the lumped inductance was about 40 microhenries.

Obviously, the invention is not limited to thef-` use of a single tuning condenser, as illustrated, but is well adapted to the various systems of push-button tuning in which one of a plurality' of pre-set trimmer condensers may be shunted across the loop by means of a suitable switching arrangement, as is well known in the art. Therefore, although the invention has been described with particular reference to the embodiment of the drawing, it will be understood that the invention is capable of various forms of physical expression, and is, therefore, not to be limited to the specific disclosure, but only by the scope of the appended claims.

I claim:

l. In a tuned loop antenna circuit, a tuning condenser, a loop antenna having a certain distributed capacity between turns and to ground, said distributed capacity being substantially equivalent to a simple shunt capacityV connected across the terminals of said loop antenna, said loop antenna and said condenser having both high and low potential terminals, connections between the low potential terminals of said loop antenna and said condenser, and additional connections between the high potential terminals of said loop antenna and said condenser, said last named connections including a lumped isolating inductance having a distributed capacity to ground which is substantially lower than that of said loop antenna, whereby said shunt capacity does not appear directly across said tuning condenser, said isolating inductance having a Q which is high compared to the Q of said loop antenna, the symbol Q designating the ratio of reactance to resistance.

2. In a tuned loop antenna circuit, a tuning condenser having predetermined maximum and minimum capacity values, a loop antenna of relativelylow Q connected inshunt relation with said condenser, said antenna having a predeterminedlimited number of turns such that the distributed capacity thereof is insufficient to limit substantially the frequency range over which the circuit maybe tuned, the inductance of said antenna alone being insufficient to enable tuning over the desired frequency range, a shield windf ing surrounding said loop antenna and tending to increase the distributed capacity of said antenna, and a. .high Q lumped inductance connected in circuit with said lojop antenna and said tuning condenser so as'to supplementthe inductance Vofsaid antenna to enable tuning over the desired-range, thesymbol Q designating the ratio of reactance to resistance, said lumped inductance having a distributed capacity which is substantially less than thatof said loop antenna.

3. In a tuned loop .antenna-circuit, a loop antenna having a certain distributed capacity to ground and a vcertain ratio of reactance to resistance, a lumped inductance connected in series with Ysaid loop vantenna,'said lumped. inductance having a substantially lower distributed capacity .to ground and a substantially higher ratio of reactance to resistance than that possessed by said loop antenna, and a-tuning condenser connected in shunt with said series connected loop andlumpedl inductance. Y

`4.. `A vtunedloop antenna 4circuit as claimed in claim 3, characterized in that the ratio of reactance to resistance of said lumped 'inductance' is at least twice the ratio of` reactance to resistance of said loop antenna.

5. In a multiple wave band radio receiver, a multiple Wave band loop antenna system comprising long-wave and short-wave elements, the long-wave element of which has a certain distributed capacity to ground and a certain ratio of reactance to resistance, a lumped inductance, means for connecting said lumped inductance in series with said long-wave element, said lumped inductance having a substantially lower distributed capacity to ground and a substantially higher ratio of reactance to resistance than that possessed by said long-wave element, a tuning condenser, and band switching means for selectively connecting said tuning condenser iny shunt with either said short-wave element or with said long-wave element and said lumped inductance.

6. In a tuned loop antenna circuit, a loop'antenna having a certain distributed capacity toV tially lower distributed capacity to ground and a substantially higher ratio of Y reactance to resistance than that possessed by said loop antenna, and a tuning condenser connected in shunt with said series connected loop and lumped inductance, said lumped inductance serving to electrically isolate the said distributed capacity of said loop antenna from said tuning condenser.

7. In a tuned loop antenna circuit, a closed series circuit comprising a loop antenna having a low Q and a relatively high distributed capacity to ground, a high Q lumped inductance having a relatively low distributed capacity to ground, the symbol Q designating the ratio of reactance to resistance, and a tuning condenser for tuning said loop and said lumped inductance to resonance with a desired signal, the inclusion of said high Q lumped inductance tending to increaseV the selectivity of said tuned loop antenna circuit.

8. In a multiple wave band radio receiver having a long-wave loop antenna and a relatively large short-wave loop antenna, a variable tuning condenser having a relati vely low maximum capacity and a relatively low'ratio of maximum to minimum capacity, switching means for selectively connecting said tuning condenser in shunt with' either one of 'said loop antennas, said longwave loopantenna having such a' relatively high eiective shunt capacity that said tuning condenser would be normally incapable of tuning said long-Wave loop antenna over the desired frequency band, and reactive means in series with said long-wave loop for isolating said shunt capacity from' said tuning condenser, said reactive means comprising a lumped inductance having a relatively low capacity to ground, whereby the capacity of said tuning condenser required to tune the receiver over the long-wave band may be relatively small, and the size of the short wave loop antenna may be correspondingly large to increase the eiciency of the receiver during short-wave reception.

9. In a multiple wave band radio receiver having a long-wave loop antenna and a relatively large short-Wave loop antenna,V a variable'tuningcondenser having a relatively low maximum sol capacity and a relatively low'ratio of maximum togr'ninimum capacity, switching means for selectively connecting said tuning condenser in shunt with either one of said loop antennas, said switching means having-long-wave and shortwave positions, said long-wave loop antenna being characterized by-having vsucha high effective shunt capacity, due to its distributed capacities, as [compared to the eiective shunt capacity of said-'short-wave loop antenna that said tuning condenser would be normally incapable of tuning said long-wave loop antenna over the desired frequency band, and aY lumped inductance having a relatively low capacity to ground, said lumped inductance being connected to said longwave loop yand said switching means so that said lumped inductance is included in the resonant loop and condenser circuit only when said switching means is in the said long-wave position.

10. In la multiple band radio receiver, a multiturn loop antenna having a certain Qand a high distributed capacity, a single-turn loop antenna having a low distributed capacity, a variable tuning condenser, switching means for selectively connecting said tuning condenser in shunt with either of said loopantennas, and reactive means in series with sai-dmulti-turn loop for isolating said distributed capacity from said tuning condenser, said reactive means comprising a lumped inductance having a greater Q than said multiturn loop antenna and having low distributed capacity, the symbol Q designating the ratio of reactance to resistance.

Y WILLIAM H. GRIMDITCH. 

